How many congressional seats are up for re-election every two years?

Every two years, all 435 seats in the U.S. House of Representatives are up for re-election. Additionally, approximately one-third of the 100 Senate seats are contested during each election cycle, resulting in about 33 or 34 Senate seats being up for re-election every two years. This means that a significant portion of congressional representation is subject to voter decision every two years.